The urea cycle spans two cellular compartments. Which two enzymes catalyze the reactions that occur inside the mitochondrion?

• A N-acetylglutamate synthetase and argininosuccinate synthetase
• B Argininosuccinate synthetase and argininosuccinate lyase
• C Ornithine transcarbamoylase and arginase
• D Carbamoyl phosphate synthetase-I and ornithine transcarbamoylase ✓

Explanation

CPS-I converts ammonia, CO2 and two ATP into carbamoyl phosphate in the mitochondrial matrix, and ornithine transcarbamoylase then transfers carbamoyl to ornithine forming citrulline, also within the matrix. Citrulline exits via a specific transporter, and the remaining three steps occur in the cytosol. Argininosuccinate synthetase, lyase and arginase are all cytosolic, so options listing them are wrong.

Reference: Lippincott Illustrated Reviews: Biochemistry, 8th ed.

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